The invention relates to an electroradiographic device comprising two electrodes which are connected to a voltage source and between which a heavy-atom rare gas is present at excess pressure, the said rare gas absorbing a substantial part of the X-radiation, a small part of a different gas being added to the said rare gas.
A device of this kind is known for example from German Offenlegungsschrift 2,258,364. This devices serves for recording X-ray images, i.e. for the recording of the intensity distribution of an X-ray beam which is incident perpendicularly to the parallel extending electrodes. An X-ray image is then formed as follows:
When X-radiation passes through the heavy atom rare gas -- preferably xenon or krypton -- present between two electrodes, the gas is ionized and the ions and electrons thus produced are accelerated in the direction of the two electrodes. One of the two electrodes is preceded by an insulating foil, for example, made of mylar, on which the charge carriers accelerated towards this electrode are incident and on which an electrical charge image is produced. This charge image is negative if the insulating foil is arranged in front of the positive electrode, while it is positive if the insulating foil is arranged in front of the negative electrode. The radiation distribution thus converted into an electrical charge image can be made visible by way of a developing method as commonly used for electrostatic copying.
For medical X-ray diagnoses it is of essential importance that the radiation dose applied to a patient during X-ray exposure is as small as possible. The sensitivity of such a device, therefore, should be as high as possible, i.e. the number of charge carriers imparted to the insulating foil per X-ray quantum should be as high as possible. One possibility of increasing the sensitivity consists in the increasing of the number of charge carriers formed per X-ray quantum absorbed by increasing the voltage between the electrodes, so that a noticeable electron multiplication occurs due to impact ionization. The number of charge carriers generated by an X-ray quantum is thus increased.
In practice, however, several drawbacks occur. In the case of a pure xenon filling at a pressure of 7 bar and a distance of 10 mm between the electrodes, a voltage of approximately 60 kV must prevail between the electrodes in order to enable the device to operate in the range of charge carrier multiplication. Moreover, uncontrolled, comparatively strong electrical discharges which disturb the charge image can occur.
Presumably in order to eliminate these drawbacks, it is stated in German Offenlegungsschrift 2,253,364 (page 10, first paragraph) that operation in the avalanche region should take place only if the product of the distance between the electrodes and the pressure is smaller than 10 mm.bar. This is because, on the one hand, the voltage to be applied can be reduced, while on the other hand no uncontrolled discharges occur. However, the quantum absorption is thus substantially reduced, which means that an inadequate fraction of the incoming X-ray quanta contributes to the image formation. This causes the so-termed "quanta" or "distribution" noise whereby the image quality is reduced. Therefore, the said publication states that operation should be in a region substantially beyond 10 mm.bar, notably in a region between 20 mm.bar and 80 mm.bar. The voltage between the electrodes must then be adjusted so that a discharge occurs in the region of the so-termed Townsend plateau, the secondary charge carriers, formed by the deceleration of the energy-rich X-ray photoelectrons, not being further multiplied.
Moreover, German Auslegeschrift 1,909,428 discloses a spark chamber containing a xenon filling which is used for the localizing detection of nuclear radiation particles, gamma or X-ray quanta, the voltage between the electrodes being chosen so that uniformly distributed spark discharges occur with a charge carrier multiplication of at least 10,000. In order to decrease the voltages to be applied to the electrodes, it is stated that between 1.05% and 6.57% diethylamine must be added to the xenon filling. Diethylamine has an ionization energy which is lower than the energy of the lowest metastable levels of the xenon atoms.
However, this spark chamber is used at an overall gas pressure of 760 Toss .apprxeq. 1 bar and a distance between the electrodes of 3.3 mm, so that a pressure/electrode distance product of .apprxeq. 3.3 mm.bar occurs. As is known already, for such a value of the pressure/electrode distance product, the absorption of the X-ray quanta by the xenon filling is so small that the higher sensitivity, in principle possible as a result of the charge carrier multiplication, cannot at all be utilized on account of the increased quanta noise.